This invention relates to a filter apparatus using a floating filter medium.
A filter apparatus is known which is designed to use a floating filter medium such as expanded polystyrene or polypropylene or polyethylene in granulated or granular form having a lower specific gravity than water to be filtered. In such a filter apparatus, the filter medium is kept in a floating layer condition and the water to be filtered is caused to flow upwardly, passing through the floating layer of filter medium for filtration. During filtering operation, the floating filter medium in granular form floats in a layer of masses to a higher level in the filter tank because of its low specific gravity, and as the floating layer is consolidated by the pressure of the streams of water caused to flow upward, the gap between individual filter material particles becomes finer to the extent that the floating layer is formed into a filter layer, through which the upward flow of water is caused to pass. Such filter layer can be washed clean by back-flow washing or flowing wash water downward therethrough.
A prior art filter apparatus using such floating filter medium has proven particularly advantageous in that back-flow washing of the filter medium can be effected only by a small amount of power which is just emough to sink the filter medium slightly despite the floating force thereof.
However, there is a problem with the prior art filter apparatus in that since the filter medium is very light in weight, if the pressure of back-flow wash water is too strong, or if the flow velocity of the back-flow wash water is out of balance with the floating-up velocity of the filter medium to the extent that the former is higher than the latter, a substantial part of the filter medium may be washed away. Conversely, if the former velocity is lower than the latter velocity, the back-flow washing operation cannot be carried out effectively. Generally, if sludge contained in the water to be filtered are caught by the layer of floating filter medium as they pass therethrough, with the result that mesh clogging is caused to the filter medium layer, a larger part of such sludge is found to have been packed closely into the filter medium layer somewhere adjacent the underside thereof to the extent that they are compactly deposited on individual particles of filter medium. For this reason, it has been difficult to separate from the filter medium such sludge deposited compactly thereon.
It is an object of this invention to provide an improved filter apparatus using floating filter medium, wherein effective means are provided for positively separating such sludge packed closely into a filter medium layer without damaging or washing away filter material particles or granules during the back-flow washing operation.
In a filter apparatus using floating filter medium, when back-flow wash water is caused to flow downward at a suitable velocity, sludge separated from the filter medium are discharged from the filter tank together with back-flow wash water since they are generally higher in specific gravity than filter medium, whereas filter medium can be retained in the filter tank in the face of the downward force of back-flow wash water because of its lower specific gravity and greater floatability. However, it has been difficult to separate sludge whose apparent specific gravity is relatively small by such downward flow of water and discharge same from the filter tank. The reason is that if back-flow wash water is caused to flow at such a high velocity as to discharge sludge having relatively low specific gravity from the filter tank, it is very difficult to accurately detect the time when sludge has been discharged and to control stopping back-flow washing quickly before filter medium is washed away.
Accordingly, in order that sludge having relatively low specific gravity can be discharged from the filter tank as well, it is necessary to separate sludge gradually from the floating filter medium layer and allow it to settle while maintaining the filter medium in floating condition and without causing the backflow wash water to flow at too high a velocity.
However, if back-flow washing is carried out at such moderate velocity only, a considerable time interval and a considerable amount of back-flow wash water are required. Moreover, with back-flow wash water at such moderate velocity, it is difficult to discharge settled sludge effectively from the filter tank.
It is another object of this invention to provide a solution to such problem and to provide a filter apparatus using floating filter medium as designed to facilitate satisfactory separation of sludge from floating filter medium during back-flow washing operation, and smooth discharge from the filter tank of sludge having relatively low specific gravity without involving loss of filter medium. More concretely, a filter apparatus uses floating filter medium wherein a filter tank is separated by a water-penetrable partition wall into an upper section forming a filtrate reservoir and a lower section forming a filter chamber, floating filter medium being placed in tne filter chamber water to be filtered being fed into the filter chamber at the bottom thereof, the water being caused to flow upwardly to pass through the floating filter medium, the resulting filtrate being removed from the filtrate reservoir. The filter apparatus being further characterized by the improvement including: an agitating blade provided at a central location in said filter chamber as viewed from above for causing upward flow of water streams, a pit provided beneath said filter chamber, a back-flow washing water discharge pipe provided in the form of a siphon-shaped pipe with a discharge opening located adjacent the bottom of the pit, said discharge pipe being provided with a siphon breaker having an air feed port positioned at a level adjacent the bottom of the filter chamber, a supply pipe having a feed opening positioned at a suitable location at or adjacent the bottom of the filter chamber and outside the pit for feeding the water to be filtered into said filter chamber.